Breaking the spectral degeneracies in black hole binaries with fast timing data

Abstract

The spectra of black hole binaries in the low/hard state are complex, with evidence for multiple different Comptonisation regions contributing to the hard X-rays in addition to a cool disc component. We show this explicitly for some of the best RXTE data from Cyg X-1, where the spectrum strongly requires (at least) two different Comptonisation components in order to fit the continuum above 3 keV, where the disc does not contribute. However, it is difficult to constrain the shapes of these Comptonisation components uniquely using spectral data alone. Instead, we show that additional information from fast variability can break this degeneracy. Specifically, we use the observed variability power spectra in each energy channel to reconstruct the energy spectra of the variability on timescales of ~10s, 1s and 0.1s. The two longer timescale spectra have similar shapes, but the fastest component is dramatically harder, and has strong curvature indicating that its seed photons are not from the cool disc. We interpret this in the context of propagating fluctuations through a hot flow, where the outer regions are cooler and optically thick, so that they shield the inner region from the disc. The seed photons for the hot inner region are then from the cooler Comptonisation region rather than the disc itself.

8 pages, 6 figures. Submitted to MNRAS

References in corpus (7)

• Modelling the behaviour of accretion flows in X-ray binaries
• Evolution of the 0.01 - 25 Hz power spectral components in Cygnus X-1
• AstroSat/LAXPC observation of Cygnus X-1 in the hard state
• Modelling hard and soft state of Cygnus X-1 with propagating mass accretion rate fluctuations
• Probing the temporal variability of Cygnus X-1 into the soft state
• Fast variability as a probe of the smallest regions around accreting black holes
• Vanishing hardness-flux correlation in Cygnus X-1: signs of the disc moving out

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